Capacitive keyboards typically have many switch keys which may be pressed and released to respectively define active and inactive switch states. Each switch key has a capacitive element that defines a low capacitance when the key is inactive and a relatively higher capacitance when the key is pressed. The change in capacitance of a key therefore indicates the operational state of the key.
Capacitive keyboards are desirable because of their structural simplicity and their associated relatively low manufacturing cost. Also, capacitive keyboards have a potentially higher reliability and a longer life than keyboards with keys that physically make and break conductive connections.
It has been suggested that a capacitive keyboard switch may utilize a spring to define a capacitance which indicates the operational state of the switch. In operation, when a pushbutton of the switch is at rest, the spring is fully extended to define a minimum capacitance with respect to an underlying dielectric sheet and stationary capacitor plate. When the pushbutton is pressed, the spring is compressed so that its coils spread out over the dielectric sheet and the underlying capacitor plate. The compressed coils of the spring are disposed adjacent to the underlying plate and therefore define an increased capacitance for as long as the pushbutton is pressed. The change in capacitance from the rest state to the depressed state is detected by a capacitance monitoring circuit which then operates assoicated switch-controlled apparatus.
A capacitive switch monitoring system can have increased reliability and greater simplicity if the switch defines a substantially zero capacitance in its rest state. The improvement in operation is due to the fact that it is easier and moe reliable to detect a change from a substantially zero capacitance to a greater capacitance, than to detect a change between two substantially nonzero values of capacitance.
Accordingly, it is an object of the invention to provide a relatively simple and reliable capacitive keyboard switch which defines a substantially zero capacitance when the switch is in its rest position and an increased capacitance when the switch is pressed.
A further object of the invention is to provide a capacitive keyboard switch with a conical conducting spring and with means for supporting the spring in spaced relation over an underlying capacitive plate, so that the spring defines a substantially zero capacitance when its associated switch pushbutton is in a rest position.
Another object of the invention is to provide a capacitive keyboard switch that is relatively easy to assemble. The switch has a capacitive spring which is trapped in slots of a switch housing and which is supported above an insulated capacitive plate of a printed circuit board. The spring has a tail portion which can be easily located for soldering to the circuit board.
A further object of the invention is to provide a quiet capacitive keyboard switch wherein the capacitive spring of the switch is supported above a printed circuit board to prevent the board from amplifying the sound of movement of the spring.